If I have 300 AD and base 100 AD that means I get + 270 damage if the ratios are switched compared to +200 damage. That's a big difference to me.

No, switching the Ratios would make you go from +200 to +180.You'd lose damage (because either way it's still gonna be the Bonus AD that gets calculated, not the whole thing.) with AD Ez and gain with AP, but the amounts aren't big enough to matter.